Text 5. Electrocardiograph

Electrical signals arising from the heart can be picked up through skin electrodes and presented on a relatively slow-moving chart as the electrocardiogram (ECG). The origin of each part of the ECG waveform is very well understood and electrocardiographic records now represent one of the commonest measurement procedures performed on patients in hospital. Abnormalities in the waveform are indicative of various types of heart disease and of cardiac distress arising from other illnesses or biochemical disturbances.

Electrocardiograms are performed in the cardiac department, and also in the wards of the hospital. The electrocardiogram is also monitored on CRT (cardioscope) screens in many wards, and in the operating theatre, but not always for the purpose of making diagnoses, more the monitoring of the condition of the patient as reflected by the strength and rate of the signals.

An electrocardiograph consists of a set of electrodes and leads which are placed on the body according to a conventional plan. The most commonly used of these electrodes are placed on the legs and arms, and others are placed in a line roughly over the heart. The electrocardiograph has a lead selector switch to configure these electrodes into different groupings to produce differential signals which represent different planes through the heart. The waveform seen at each of the switch positions is slightly different.

Electrocardiographs are normally portable or mounted on light trolleys so that they can be moved around the hospital, and are often battery operated. The most common instruments record one lead configuration at a time, although three channel versions are becoming common. The ECG signal is normally about 1 mV in amplitude, and with frequency components from less than 1 Hz to 100Hz. These are amplified in a differential amplifier having a frequency response of typically 0.02 to 100 Hz, and very high common-mode rejection ratio (CMRR) to cut down the a.c. interference picked up from the mains supply by the long electrode leads. A standard paper speed of 25 mm/s and sensitivity of 10 mm/mV is used for most recordings and the amplitudes of the various parts of the waveform are usually quoted in millimetres.

A typical ECG recording procedure would consist of cleaning the skin, braiding, attaching electrodes and applying the conductive electrode jelly, and with the patient relaxed, recording a few centimetres of the ECG at each of the standard lead positions. These are then sent for diagnostic reporting by a cardiac physician. The foetal ECG signal is also sometimes monitored during pregnancy and labour to record the foetal heart rate.

Text 6. Electrode

To record the ECG, EEG, EMG, etc. electrodes must be used as transducers to convert an ionic flow of current in the body to an electronic flow along a wire. These are usually made of metal. Two important characteristics of electrodes are electrode potential and contact impedance. Good electrodes will have low stable figures for both of the above characteristics.

Electrode potential arises because a metal electrode in contact with an electrolyte (body fluids) forms a half cell with a potential dependent upon the metal in use and the ions in the electrolyte. One might expect that two electrodes of the same material would produce the same electrode potential which would cancel out in any recording, but the actual potentials do depend upon the conditions of contact; for instance, if two steel electrodes are placed in contact with the skin there may be a net contact potential between them of 100 mV. This might cause serious problems when amplifying signals in the microvolt region.

The most widely used electrodes for biomedical applications are silver electrodes which have been coated with silver chloride by electrolysing them for a short time in a sodium chloride solution. When chlorided the surface is black and has a very large surface area. A pair of such electrodes might have a combined electrode potential below 5 mV.

All electrodes suffer from variations in contact resistance due to movement, and the drying out of any coupling medium. This is improved by setting the electrode back slightly from the surface of the skin (floating electrode) on a quantity of coupling jelly (electrolyte paste). A further problem may arise if there is any direct current flowing through the electrode arising from faulty equipment or from small (microamp) bias currents in the measuring amplifier circuit. Over a period of time these currents cause chemical changes at the surface of the electrode causing polarization with consequent increase in the electrode potential. This may cause drift of the electrode potential and damage to the skin due to the chemical action. Many types of recording electrodes exist including metal discs, needles, suction electrodes, glass microelectrodes, foetal scalp clips or screws, etc.

Electrodes are also used to inject electricity into the body as in faradism, TENS, surgical diathermy and physiotherapy diathermy. Electrodes also exist in some analytical apparatus to measure the concentration of specific ions.